This invention relates to multi-piece solid golf balls featuring a high rebound and pleasant feel when hit.
Prior art solid golf balls include multilayer structure golf balls in which the core and/or the cover is formed of multiple layers as disclosed in JP-A 8-336617, 8-336618, 9-56848, 9-299510, 11-417, and 11-4916. 
The solid golf balls are formed into a multilayer structure for the purposes of acquiring an equivalent feel to wound golf balls and improving distance. In general, making much of feel often fails to meet the distance requirement whereas satisfying distance often leads to a poor feel. There is a desire to have a golf ball which finds a better compromise between distance and feel.
An object of the invention is to provide a solid golf ball which is resilient enough to increase the distance and gives a pleasant feel when hit.
The invention pertains to a multi-piece solid golf ball having at least three layers. It has been found that when a layer comprised of a thermoplastic resin in admixture with rubber powder is provided other than the cover, there is obtained a solid golf ball which is softer and more resilient than prior art golf balls. More improvements are made when a highly resilient thermoplastic resin having a low glass transition temperature is admixed with a sufficient amount of a hard rubber powder obtained from a rubber composition having a relatively high content of zinc diacrylate.
In one aspect, the invention provides a solid golf ball comprising a core, an intermediate layer, and a cover wherein the intermediate layer is comprised of a thermoplastic resin in admixture with rubber powder.
In another aspect, the invention provides a solid golf ball comprising a core, a mantle layer, an intermediate layer, and a cover wherein the mantle layer and/or intermediate layer is comprised of a thermoplastic resin in admixture with rubber powder.
In one preferred embodiment, the thermoplastic resin is a polyester resin, polyurethane resin, polyurethane ester resin, polyamide elastomer or a mixture thereof, and has a glass transition temperature (Tg) of up to xe2x88x9210xc2x0 C. and a resilience of at least 50. The thermoplastic resin and the rubber powder are preferably admixed in a weight ratio of from 75:25 to 25:75. The rubber powder is preferably obtained by vulcanizing a rubber composition comprising 100 parts by weight of polybutadiene and 30 to 45 parts by weight of zinc diacrylate. The rubber powder preferably has a mean particle size of 0.7 to 3 mm.
The solid golf ball of the invention has a core, an intermediate layer enclosing the core, and a cover enclosing the intermediate layer. In the preferred solid golf ball, a mantle layer intervenes between the core and the intermediate layer.
The core may be formed of a rubber composition, a thermoplastic resin or a mixture thereof. The rubber composition may be of any desired formulation, preferably a rubber composition comprising polybutadiene as a base. The polybutadiene used herein is preferably 1,4-cis-polybutadiene containing at least 40% of cis structure. In the base rubber, natural rubber, polyisoprene rubber or styrene-butadiene rubber may be blended with the polybutadiene if desired. Increasing the rubber component is effective for improving the rebound of golf balls.
In the rubber composition, a crosslinking agent may be blended with the rubber component. Exemplary crosslinking agents are zinc and magnesium salts of unsaturated fatty acids such as zinc dimethacrylate and zinc diacrylate, and esters such as trimethylpropane methacrylate. Of these, zinc diacrylate is preferred. The crosslinking agent is preferably used in an amount of at least about 10 parts and up to about 40 parts by weight per 100 parts by weight of the base rubber.
Also a vulcanizing agent is generally blended in the rubber composition. Organic peroxides are preferred vulcanizing agents. Such peroxides are commercially available, for example, under the trade name of Percumyl D and Perhexa 3M from Nippon Oil and Fats K.K. The vulcanizing agent may be blended in an amount of at least about 0.6 part and up to about 2 parts by weight per 100 parts by weight of the base rubber.
If necessary, antioxidants and specific gravity adjusting fillers such as zinc oxide and barium sulfate are blended in the rubber composition.
The core may be either a single core or a multilayer core of two or more layers. In the latter case, for example, an inner core formed of a rubber composition is enclosed with an outer core formed of a thermoplastic resin, typically ionomer resin.
Preferably, the core has a diameter of at least 25 mm, especially at least 30 mm and up to 39 mm, especially up to 37 mm. The core has a deflection of at least 2.0 mm, especially at least 2.5 mm and up to 5.0 mm, especially up to 4.5 mm, when a load of 100 kg is applied thereto. With the diameter and deflection under 100-kg load of the core set within the above ranges, the purpose of improving the rebound and feel of golf balls at the same time is more effectively accomplished.
According to the invention, the intermediate layer is formed around the core. The purpose of the invention is ore effectively accomplished by enclosing the core with the mantle layer and enclosing the mantle layer with the intermediate layer.
The mantle and intermediate layers each are formed of thermoplastic resins. Useful thermoplastic resins include polyurethane resins (thermoplastic polyurethane elastomers), polyester resins (thermoplastic polyester elastomers), polyurethane ester resins, thermoplastic polyamide elastomers, and ionomer resins. They are commercially available under the trade name of Pandex (thermoplastic polyurethane resins by Dai-Nihon Ink and Chemicals K.K.), Hytrel (thermoplastic polyester resins by Dupont-Toray K.K.), Surlyn (ionomer resins by Dupont), Himilan (ionomer resins by Dupont-Mitsui Polychemical K.K.), Rilsan (polyamide resins by Dupont-Toray K.K.), and Pebax (thermoplastic polyamide elastomers by Toray K.K.).
In the solid golf ball of the invention, either one or both of the mantle layer and the intermediate layer are formed of a thermoplastic resin in admixture with rubber powder. The thermoplastic resin used herein is preferably a polyester resin, polyurethane resin, polyurethane ester resin, polyamide elastomer or a mixture thereof. These thermoplastic resin should preferably have a glass transition temperature (Tg) of up to xe2x88x9210xc2x0 C., more preferably up to xe2x88x9230xc2x0 C., and especially up to xe2x88x9240xc2x0 C., and a resilience of at least 50, more preferably at least 60. A thermoplastic resin having a higher Tg and a lower resilience would fail to provide the ball with good rebound.
The rubber powder to be admixed with the thermoplastic resin may be formed of a rubber composition similar to the above-described core-forming rubber composition. Especially preferred is a rubber composition comprising polybutadiene, especially 1,4-cis-polybutadiene having at least 40% of cis structure as a rubber component and zinc diacrylate as a crosslinking agent. It is recommended to use a hard rubber powder obtained from a rubber composition comprising 100 parts by weight of polybutadiene and about 30 to 45 parts, especially about 30 to 40 parts by weight of zinc diacrylate by vulcanizing at about 150 to 170xc2x0 C. for about 10 to 30 minutes.
The rubber powder preferably has a mean particle size of at least 0.7 mm, especially at least 1.0 mm and up to 3.0 mm, especially up to 2.0 mm. A rubber powder having a too small particle size has an increased interfacial surface area which can be detrimental to resilience. Although a mean particle size in excess of 3.0 mm is not detrimental to ball performance, it is recommended for effective molding, especially injection molding to restrict the mean particle size to 3.0 mm or less.
Preferably the thermoplastic resin and the rubber powder are admixed in a weight ratio of from 75:25 to 25:75, more preferably from 60:40 to 25:75, most preferably from 60:40 to 30:70. A mixing ratio outside the range sometimes fails to achieve the object of the invention.
In the admixed layer of thermoplastic resin and rubber powder, an inorganic filler, plasticizer and other addenda are blended if necessary.
The thermoplastic resin in the admixed layer should preferably have a Shore D hardness of at least 20, especially at least 25 and up to 55, especially up to 50, as measured alone, that is, with the rubber powder removed.
In an embodiment of the invention having both the mantle layer and intermediate layer either one of which does not contain the rubber powder, the rubber powder-free layer should preferably have a Shore D hardness of at least 55, especially at least 60 and up to 70, especially up to 68, and higher than the hardness of the admixed layer.
Preferably the mantle layer has a gage (or radial thickness) of at least 0.5 mm, especially at least 0.8 mm and up to 3.0 mm, especially up to 2.5 mm. The intermediate layer preferably has a gage of at least 1.0 mm, especially at least 1.5 mm and up to 7.0 mm, especially up to 5.0 mm when the mantle layer is present; and a gage of at least 1.0 mm, especially at least 1.5 mm and up to 8.0 mm, especially up to 6.0 mm when the mantle layer is absent.
The cover may be formed of thermoplastic resins including ionomer resins and thermoplastic elastomers, with the ionomer resins being preferred. The cover should preferably have a Shore D hardness of at least 40, especially at least 45 and up to 70, especially up to 65, and yet higher than the hardness of the admixed layer. Also the cover should preferably have a gage of at least 0.5 mm, especially at least 0.8 mm and up to 3.0 mm, especially up to 2.5 mm. Additives such as inorganic fillers are optionally blended in the cover.
Like ordinary golf balls, the golf ball of the invention is provided on its surface with 300 to 600 dimples in a well-known arrangement.
The diameter and weight of the golf ball should comply with the Rules of Golf. Typically the golf ball has a diameter of not less than 42.65 mm, especially not less than 42.67 mm and not greater than 42.75 mm and a weight of not greater than 45.93 g, especially not greater than 45.90 g and not less than 45.10 g.